Magnesium hydride (MgH2) is a promising material for hydrogen storage applications because of its high gravimetric (7.6 wt.%) and volumetric (110 kg m-3) densities and low cost. However, there are some drawbacks limited practical application of this hydride. First of all, it is too stable thermodynamically (H=-75 kJ/mol). It results in high desorption temperature (>400ºC). Moreover, the MgH2 exhibits relatively poor absorption/desorption kinetics below 350ºC. Recently, it has been shown that the sorption kinetics of MgH2 can be dramatically improved by ball milling of MgH2 with some transition metals and their oxides or halides. Among them, NbF5 additive seems most significantly enhance the sorption kinetics of MgH2. However, it is still not clear how the NbF5 promotes the dehydrogenation and hydrogenation reactions. Additionally, there is a lack of information about microstructure and sorption properties of MgH2 with nanosized NbF5 after cycling loading.
In our work we present the effect of cycling on microstructure and hydriding-dehydriding properties of nanocrystalline MgH2 with nanosized NbF5 prepared by mechanical milling. Commercial MgH2 powder was mixed with NbF5 powder (7 wt.%) and subsequently ball milled in an inert atmosphere in a planetary mill. The phase structure, morphology and chemical composition were investigated by XRD, SEM, EDS and DSC-TG. The hydrogen sorption properties and pressure composition isotherms were evaluated using a volumetric Sievert apparatus and temperature programmed desorption. Our results have shown a considerable catalytic effect of NbF5 additive on both the dehydrogenation temperature and hydriding/dehydriding kinetics of MgH2. The obtained nanocomposite exhibits good reversibility in the pressure composition isotherms at 325ºC. However, degradation of nanostructure and hydrogen storage capacity after prolonged cycling are observed, apparently related to the grain growth during cycling at elevated temperature.